1. Field of the Invention
The invention relates to a direct conversion receiver, and in particular, to DC offset cancellation in a direct conversion receiver.
2. Description of the Related Art
FIG. 1a shows a conventional direct conversion receiver with mismatch calibration. An RF signal is received through an antenna 102, and the RF module 104 performs a preliminary adjustment such as low noise amplification (LNA) and bandpass filtering. The mixer 106 then down converts the RF signal to a baseband signal, and the filtering module 108 performs a post adjustment such as low pass filtering (LPF) and programmable gain amplification (PGA) to generate a quality baseband signal before sending to the analog to digital converter (ADC) 110. DC offset is a common issue induced in direct conversion receivers, degrading down conversion performance. In some cases, a calibrator 112 is provided to calibrate component imbalances in the mixer 106. The calibrator 112 may be coupled to the mixer 106, adjusting component mismatches such as resistor imbalance according to DC offset measured from the output of mixer 106 or the filtering module 108.
FIG. 1b shows a conventional mixer with an adjustable differential loading pair 120. As known, DC offset of a mixer 106 is induced from component imbalances possibly occurring in the first switch 126, second switch 128, and the transconductance stage 130. The differential loading pair 120 comprises a first resistor 122 and second resistor 124, with at least one an adjustable resistor. The mismatch of differential loading pair 120 can be adjusted to minimize the induced DC offset, inducing an optimal mixer output. Thus, the calibrator 112 operates in a calibration mode to adjust the first resistor 122 or second resistor 124 through an adjustment value #adj. When a specific resistor imbalance is found to correspond to the optimal mixer output, the calibrator 112 configures the differential loading pair 120 with that specific resistor imbalance value, and the direct conversion receiver switches to a normal mode, operating with the adjusted mixer 106. When the direct conversion receiver operates in the normal mode, the calibrator 112 is turned off or removed. Typically, the calibrator 112 is only provided in the manufacturing stage to characterize every mixer 106 in the product line, and each mixer 106 may be configured with different adjustment value #adj in the calibration due to component differences. With a calibrated mixer 106, a down conversion receiver can operate with optimum performance.
FIGS. 2a and 2b show various implementations of the filtering module 108. In FIG. 2a, three DC offset cancellation (DCOC) loops are shown. The filter 204 and the first DC offset canceller 200 form a first DCOC loop, the filter 212, amplifier 214 and second DC offset canceller 210 form a second DCOC loop, and the amplifier 222 and third DC offset canceller 220 form a third DCOC loop. Conventionally, the DC offsets are cancelled stage by stage, each consuming a predetermined convergence time. There are various implementations of the DC offset cancellers 200, 210 and 220. The DC offset cancellers 200, 210 and 220 may be implemented in analog or digital form, and the convergence speed may be fast or slow. FIG. 2b shows another known implementation of filtering module 108. The DC offset canceller 230 forms a DCOC loop with amplifier 232, filter 234 and filter 236 while the DC offset canceller 240 detects and cancels DC offset of the amplifier 242. Typically, the DC offset cancellers 200 to 240 perform DC offset cancellation by measuring DC offsets from output ends of their loops, and generating compensations to the input of their input ends.
IEEE paper “Characterization of IIP2 and DC-Offsets in Transconductance Mixers”, disclose how IIP2 can be calculated as functions of load resistor imbalance and duty cycle mismatch, and the resistor imbalance is tuned to optimize the IIP2 of a mixer. The mixer output tuned by the resistor imbalance may comprise a DC offset comprising static and dynamic parts:
                              V          DC                =                                            V              DC_static                        +                          V                              DC_dynami                ⁢                c                                              =                                                    R                L                            ⁢                              I                T                            ⁢                                                Δ                  ⁢                                                                          ⁢                  R                                2                                      +                                          1                2                            ⁢                              R                L                            ⁢                              η                nom                            ⁢              gm              ⁢                                                          ⁢                              α                2                ′                            ⁢                                                A                  RF                  2                                ⁡                                  [                                                            Δ                      ⁢                                                                                          ⁢                                              η                        ⁡                                                  (                                                                                    Δ                              ⁢                                                                                                                          ⁢                              gm                                                        +                                                          Δ                              ⁢                                                                                                                          ⁢                                                              A                                RF                                                                                                              )                                                                                      +                                          Δ                      ⁢                                                                                          ⁢                                              R                        ⁡                                                  (                                                      1                            +                                                          Δ                              ⁢                                                                                                                          ⁢                              gm                              ⁢                                                                                                                          ⁢                              Δ                              ⁢                                                                                                                          ⁢                                                              A                                RF                                                                                                              )                                                                                                      ]                                                                                        (        1        )            
Where ΔARF is amplitude difference of the RF signals VRF+ and VRF−; gm is conductivity of the components in first switch 126 and second switch 128, and Δgm is their mismatch; Δη is duty cycle mismatch of the local oscillation signals VLO+ and VLO−, ΔR is the resistor imbalance of the first resistor 122 and second resistor 124. By calibrating the mixer 106 with calibrator 112, the dynamic part can be eliminated by assigning the resistor imbalance ΔR to a specific value, however, the static part still remains and is sent to the filtering module 108.
                              V                      DC_dynami            ⁢            c                          =                              1            2                    ⁢                      R            L                    ⁢                      η            nom                    ⁢          gm          ⁢                                          ⁢                      α            2            ′                    ⁢                                    A              RF              2                        ⁡                          [                                                Δ                  ⁢                                                                          ⁢                                      η                    ⁡                                          (                                                                        Δ                          ⁢                                                                                                          ⁢                          gm                                                +                                                  Δ                          ⁢                                                                                                          ⁢                                                      A                            RF                                                                                              )                                                                      +                                  Δ                  ⁢                                                                          ⁢                                      R                    ⁡                                          (                                              1                        +                                                  Δ                          ⁢                                                                                                          ⁢                          gm                          ⁢                                                                                                          ⁢                          Δ                          ⁢                                                                                                          ⁢                                                      A                            RF                                                                                              )                                                                                  ]                                                          (        2        )                                          V          DC_static                =                              R            L                    ⁢                      I            T                    ⁢                                    Δ              ⁢                                                          ⁢              R                        2                                              (        3        )            